Seismic imaging plays an important role in the study of underground formations, and in particular in the study of underground formations that are related to subsurface reservoirs containing e.g. fresh water, gas hydrates or hydrocarbons.
One way of acquiring seismic data is by disposing a plurality of sensors on the earth's surface. These sensors may be disposed on land, on the seabed, or in a land/sea transition zone. Furthermore by deploying sensors that measure the particle velocity in three orthogonal directions as well as the local pressure variations of an elastic wave phenomenon passing the sensors so called multi-component seismic data can be acquired. The elastic wave phenomenon is commonly generated by a firing a pressure source gun at the sea-surface or by vibrating the earth with a large mass.
By monitoring the seismic energy reflected from underground formations with the multi-component sensor network, information on the underground itself can be deduced. One specific benefit of multi-component seismic measurements is that it holds more information about the underground formations than so-called conventional seismic that measures only one component.
In a marine exploration setting, two modes of seismic waves are typically considered:    1. Seismic energy that propagates as a longitudinal (or pressure) wave from the source into the underground; where it is reflected at a formation and then travels again as longitudinal wave to the sensor network. This mode is also denoted as PP mode.    2. Seismic energy that propagates as a longitudinal (or pressure) wave from the source into the underground; where it is reflected and converted to a transversal (or shear) wave mode and propagates as this to the sensor network. This mode is also denoted as PS mode.Since the propagation velocity is different for the longitudinal wave mode and each of the two possible transversal wave modes, a reflecting formation in the subsurface is recorded at different times at the sensor network depending on the mode. Furthermore, the amplitudes of the recordings are different since a reflection of a pressure wave is governed in another way than the reflection and conversion of a pressure mode into a shear mode.
Conventionally, the acquired PP and PS seismic data are subjected to separate processing sequences, each addressing the special nature of its mode. To those acquainted in the art of seismic processing it is clear that the outcome is typically a time-migrated PP and a time-migrated PS image volume of the underground where the vertical (depth) axis of the image volume is denoted in recording time rather than in real depth measured in meters or feet.
In order to facilitate a joint analysis of the PP and PS seismic image volumes, these data have to be transferred to a common domain in a further processing step.
Typically the PS seismic image volume is stretched vertically to the time scale of the PP seismic image volume by a process of event correlation. A state of the art embodiment of this process consists of:    1. Interpreting a first set of horizons on the PP seismic image volume.    2. Interpreting a second set of horizons on the PS seismic image volume where each horizon of the second set corresponds to the same reflecting subsurface event identified as the corresponding horizon in the first set.    3. Stretching the PS image volume to PP time scale by displacing samples at the location of the PS horizons to the location of their corresponding PP horizons. The samples at locations in between two PS horizons are displaced by an amount found by interpolating the displacement between the horizon locations.
The above procedure of event correlation has a number of disadvantages. It is work intensive since an interpretation of this kind has to be produced by a highly skilled person who identifies the same reflecting event in both PP and PS data. Thus it may involve a considerable manual effort. Furthermore since the event correlation is only based on seismic data at the location of the interpreted horizons only a part of the available information is exploited. Finally, the time interval in between two neighboring horizons is commonly rather large and interpolation of the displacement at intermediate positions becomes inaccurate. It should also be noted that although the word “stretching” is used here and throughout the application, this process can alternatively be thought of as “squeezing” because shear transmission mode velocities are typically lower than compressional transmission mode velocities.
To overcome some of these disadvantages Fomel (see Fomel, S. and Backus, M., 2003, Multicomponent seismic data registration by least squares, 73rd Ann. Internat. Mtg.: Soc. of Expl. Geophys., 781–784) recently proposed a scheme to automatically refine a displacement estimate obtained by manual event correlation applying a warping scheme on the seismic signal. The method is used to generate a high-resolution vp–vs ratio estimate. It has however some crucial deficiencies that will lead to non-robust results for typical real data cases. First of all, in addition to the time difference between corresponding PS and PP samples also an amplitude-scaling factor between said samples is estimated. Though this amplitude-scaling factor in general reflects the physical differences between the PP and PS seismic reflectivity, its particular implementation renders the algorithm less robust. In principle, it is possible to apply only amplitude scaling in order to transfer a PS seismic to the PP seismic i.e. without displacing the PS seismic along the time-axis. Consequently the inversion to two variables i.e. the amplitude scaling and the displacement along the time axis is non-unique and leads to robustness problems in the algorithm. Fomel recognizes this and states that “to avoid being trapped in a local minimum the method needs a good initial guess for the warping function w(t)”. Furthermore, Fomel's method operates only on individual signals and thus does not exploit the lateral correlation inherent in seismic data.
Kristiansen et al. (see Kristiansen, P. and Van Dok, R., 2003, Event registration and Vp/Vs correlation analysis in 4C processing, 73rd Ann. Internat. Mtg.: Soc. of Expl. Geophys., 785–788) proposed an alternative scheme to arrive at a high-resolution vp–vs ratio estimate by scanning over possible vp–vs ratios and picking local maxima in a semblance panel. Yet, as with Fomel's method, an initial vp–vs ratio estimate produced by manual interpretation is needed.
The present invention overcomes the deficiencies with the present methods and produces a high-resolution vp–vs ratio estimate without the need of manual interpretation.
Accordingly, it is an object of the present invention to provide an improved method of correlating seismic events associated with different types of seismic transmission modes and for deriving and using information resulting from such correlations, such as estimates of vp–vs ratios.